Mass-Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H<inf>2</inf>O, Na, and K

Atmospheric compositions can provide powerful diagnostics of formation and migration histories of planetary systems. We investigate constraints on atmospheric abundances of H $2$ O, Na, and K, in a sample of transiting exoplanets using latest transmission spectra and new H $2$ broadened opacities of Na and K. Our sample of 19 exoplanets spans from cool mini-Neptunes to hot Jupiters, with equilibrium temperatures between $\sim $300 a n d 2700 K . U s i n g h o m o g e n e o u s B a y e s i a n r e t r i e v a l s w e r e p o r t a t m o s p h e r i c a b u n d a n c e s o f N a, K, a n d H$ _2 $O, a n d t h e i r d e t e c t i o n s i g n i f i c a n c e s, c o n f i r m i n g 6 p l a n e t s w i t h s t r o n g N a d e t e c t i o n s,6 w i t h K, a n d 14 w i t h H$ _2 $O . W e f i n d a m a s s − m e t a l l i c i t y t r e n d o f i n c r e a s i n g H$ _2 $O a b u n d a n c e s w i t h d e c r e a s i n g m a s s, s p a n n i n g g e n e r a l l y s u b s t e l l a r v a l u e s f o r g a s g i a n t s a n d s t e l l a r / s u p e r s t e l l a r f o r N e p t u n e s a n d m i n i − N e p t u n e s . H o w e v e r, t h e o v e r a l l t r e n d i n H$ _2 $O a b u n d a n c e s, f r o m m i n i − N e p t u n e s t o h o t J u p i t e r s, i s s i g n i f i c a n t l y l o w e r t h a n t h e m a s s − m e t a l l i c i t y r e l a t i o n f o r c a r b o n i n t h e s o l a r s y s t e m g i a n t p l a n e t s a n d s i m i l a r p r e d i c t i o n s f o r e x o p l a n e t s . O n t h e o t h e r h a n d, t h e N a a n d K a b u n d a n c e s f o r t h e g a s g i a n t s a r e s t e l l a r o r s u p e r s t e l l a r, c o n s i s t e n t w i t h e a c h o t h e r, a n d g e n e r a l l y c o n s i s t e n t w i t h t h e s o l a r s y s t e m m e t a l l i c i t y t r e n d . T h e H$ _2$O abundances in hot gas giants are likely due to low oxygen abundances relative to other elements rather than low overall metallicities, and provide new constraints on their formation mechanisms. The differing trends in the abundances of species argue against the use of chemical equilibrium models with metallicity as one free parameter in atmospheric retrievals, as different elements can be differently enhanced.

Keywords

astro-ph.EP, astro-ph.EP, astro-ph.SR

Journal Title

Astrophysical Journal Letters

Journal ISSN

2041-8205
2041-8213

Volume Title

887

Publisher

American Astronomical Society

Publisher DOI

https://doi.org/10.3847/2041-8213/ab5a89

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Attribution 4.0 International

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Science and Technology Facilities Council (ST/N000927/1)

Collections

Cambridge University Research Outputs

Mass-Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H2O, Na, and K

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Peer-reviewed

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